Recent years have seen that the existing incandescent light bulbs and fluorescent lamps are being replaced at a rapid pace by light-emitting devices, such as white light emitting diodes that are obtained by combining semiconductor light-emitting elements, which are used for blue light emitting diodes, and yellow phosphor that emits yellow fluorescence. These light-emitting devices have achieved increase in efficiency to have power-to-light conversion efficiency exceeding 100 lm/W and have been made available in various sizes, such as with a package size of 5 mm2 to 100 mm2, at reasonable prices. As a result, the light-emitting devices come into wide use rapidly.
On the other hand, there is a problem of poor color rendition, because the light emitted from these white light emitting diodes is pseudo white light obtained by combining blue light and yellow light.
In view of this, for example, Patent Literature (PTL) 1 and 2 present a light-emitting device in which, in addition to a first phosphor that emits yellow right with blue semiconductor light-emitting device that emits blue light, second phosphor, which emits red light, mixed with the first phosphor is applied on the blue semiconductor light-emitting element. The following describes conventional light-emitting devices with reference to FIG. 16 and FIG. 17.
In this conventional example, a first phosphor particle is a phosphor particle of yttrium aluminum garnet type, and a second phosphor particle is a phosphor particle including CaAlSiN3 crystal activated with Eu.
A light-emitting device 1021 that is of a chip type white light emitting diode lamp includes a white alumina ceramics substrate 1029 having high reflectance against visible light. Two lead wires 1022 and 1023 are fixed to the alumina ceramic substrate 1029, and one end of each of the wires is located in a substantially center area of the substrate, and the other end of each of the wires extends to outside to serve as an electrode to be soldered at the time of mounting to an electric board. A blue light emitting diode element 1024 is placed and fixed on one end of the lead wire 1022 in the center area of the substrate. A lower electrode of the blue light emitting diode element 1024 and the lead wire 1022 below the lower electrode are electrically connected with conductive paste, and an upper electrode and the lead wire 1023 are electrically connected through a thin gold wire 1025.
Phosphor 1027, which is obtained by mixing a first resin and second phosphor, is dispersed in first resin 1026, and is provided near the blue light emitting diode element 1024. The first resin 1026 including the dispersed phosphor 1027 is transparent, and covers the entire blue light emitting diode element 1024. Furthermore, a wall member 1030, which is a white silicone resin, is fixed on the alumina ceramics substrate 1029. The wall member 1030 is in a shape including a hole in the center area, and includes a tilted face having a curved shape at a portion facing the center. The titled face is a reflector for guiding light toward the front. The hole in the center area of the wall member 1030 is a recess in which transparent second resin 1028 is filled to seal all the blue light emitting diode element 1024 and the first resin 1026 including the dispersed phosphor 1027. The first resin 1026 and the second resin 1028 include the same epoxy resin. With this structure, a spectrum indicated by a solid line shown in FIG. 17 is obtained from the light-emitting device 1021, and the chromaticity coordinates of x=0.338, and y=0.330 are obtained.
However, the light-emitting device according to the conventional technique has the following problem. Specifically, in the spectrum shown in FIG. 17, light emission in red to infrared region exists. However, as indicated by a visibility curve shown as a dotted line, human eyes have low sensitivity to the light having a wavelength greater than or equal to 680 nm, and thus such light does not contribute to the brightness of the light-emitting device. Furthermore, for the light in this region, Stokes shift at the time of conversion from blue light, which is excitation light, is great. Consequently, the conversion loss is significant. In view of this, PTL 2 presents a light-emitting device having a structure that uses quantum dot phosphor which can control the peak wavelength and has a narrow full-width at half-maximum in the spectrum, to achieve efficient controlling of light spectrum in the red to infrared region.